Bridge Deck Removal & Falsework

[facca]

We are a contractor on a small bridge project which requires a 7" bridge deck slab to be removed & replaced between the existing cast-in-place girders.The bid design shows a temporary support structure at mid span of these existing girders with no stated bearing requirement.My falsework engineer has stated that we do not require a support at mid span due to the fact that the new deck is similar to the replaced one and that if it supported the old deck they will support the new deck.We have asked the designer to provide us with the bearing requirements for these supports and they have not been provided after 5 weeks and repeated requests.What is our next step?

 

[TTK]

It took me a minute to understand the problem, but I beleive you are saying the falsework shown on the bid documents was to support the girders and not the wet concrete for the deck slab.

I assume the bridge is closed to traffic for this work and we can ignore live load considerations.

If my assumptions above are correct, then the designer is simply asking for these existing cast-in-place girders to be shored during the replacement of the deck slab. If the girders are shored at midspan, much less stress will be put into them from the wet concrete for the new deck slab.

After the concrete deck has cured, the shoring can be removed and much more composite capacity will remain for the live load than if the shoring did not occur. Even if the original girders were not shored during construction of the original deck, by shoring for the replacement, the live load capacity can be improved somewhat.

The shoring also prevents excessive deflections at midspan which could also have been a design consideration.

I hope your bid price included enough for the shoring, and the shoring should be designed to support the weight of the wet concrete, reinforcing, forms, screeding equipment etc...

The shored girders act as a two-span continuous beam so make sure the reaction used for the design of the shoring towers takes this effect into account.

Good luck, and give us an update on the falsework used once it is designed.

 

[SlideRuleEra]

What you have described sounds like an old-fashioned bridge design with deck girders. When the exisiting deck is removed you are weakening the load carrying capability of the girder. The reduced depth of the integral deck/girder lowers both the section modulus (strength) and moment of inertia (deflection) of the girder that remains. (As an analogy, think of what removing the top flange of a load carrying steel wide flange beam would to it's strength.)

Recommendation: Provide the temporary support sturcture that you bid on. You have a lot at risk if something goes wrong.

 

[Zambo]

As TTK states you need shoring otherwise it is possible that the beans will deflect during casting of the new deck slab and the final load bearing capacity of the bride will be affected. Of course a design check may be able to show that the deflection is negligible.

I worked on a project like this before and we used twin steel plate girders supported on the bridge piers with a central underslung supporting beam. This worked well but as it was a river barrage withe 43 spans we had good re-use of the steelwork. Incidently even though we proved after a couple of spans that deflections were negligible the design consultant would not relax the requirement for a central support.

Zambo

 

[Isntbard]

The bridge was probably origionaly designed with the assumption that the girders and slab were supported by falsework that was left in place until the slab concrete reached its design strength. When the falsework was removed the stress in the reinforcing steel in the bottom of the beams would be calculated using the slab as the compression flange. If you assume the slab is supported by the beams with out falsework, then the stresses would be calculted using only the top of the beam as the compression flange. This will result in much higher stresses.

 

[cage]

Even if the beams were not designed as composite, it may be that support is required because lateral support (of the deck) has been removed.